The present invention relates generally to the casting of molten metal, and particularly to sensing the level of molten metal in an opaque container that supplies an inert, direct chill casting process with molten metal.
When casting in an inert atmosphere, such as when casting aluminum-lithium alloys, the mold containing the molten metal for withdrawal and casting is sealed in order to contain the inert atmosphere. The interior of the mold is thus not visible from the outside, so that one might view the level of metal within the mold. It is necessary to know the amount of metal within the mold so that the casting process can take place without interruption because of insufficient metal in the mold. It is therefore necessary to use a level sensing system that can "see" the level of molten metal and provide an accurate measurement of the level.
One such sensing system employs eddy currents. However, eddy current sensing devices require close access to the molten metal since the sensing range of such devices is not large. Beyond a certain minimum range, the output of such devices becomes nonlinear and unstable. Preferably, the eddy current sensor should see no metal except that of the metal being examined, which in the case of inert, direct chill casting, which uses a completely enclosed steel outer shell, makes the eddy current sensing enterprise problematic.
Ultrasonic sensing devices have appropriate sensing ranges but are unstable and unreliable because of the adverse affects of thermal gradients on sound waves. It can be appreciated that the molten metal within a mold is extremely hot, whereas in progressing to the exterior of the mold, one encounters generally a room temperature condition.